This invention relates to coated abrasives and more specifically to grinding aid formulations that give particularly advantageous results when used with coated abrasive products.
Typically a coated abrasive product is formed by depositing an abrasive grit on a substrate, which is usually a flat sheet, belt, disc or the like, and adhering the grit to the surface using a "maker" coat. Over the top of this layer, a further layer called a "size" coat is applied to enhance the adhesion of the grit to the substrate. Occasionally it is desirable to add a further coat on top of the size coat incorporating a grinding aid to improve the grinding performance when the product is in use. This coat is commonly called the "supersize" coat and it is understood that "super-" here connotes location rather than quality.
The various layers are usually based on a polymeric binder material that, upon curing, forms a continuous film. This polymer can be selected from phenolic resins, radiation curable polymers, epoxy resins, polyurethanes and the like. The most common binder used is a phenolic resin. Frequently the same basic resin is used for all the layers since this ensures a degree of compatibility between contiguous layers.
The use of a coated abrasive product to grind a metal substrate results in the generation of a good deal of heat. As a result the grits may become dulled, the workpiece may be burned or the grits can be dislodged from the coated abrasive. Sometimes all three negative consequences ensue. It has been found that certain materials, called grinding aids, enhance the ease with which the cutting action occurs and hence prolong the cutting life of the coated abrasive. The reason for this improvement is not fully understood. Various theories have been proposed to explain the observation postulating chemical interactions or cooler cutting conditions. Whatever the truth of the matter, this enhancement is usually found only with relatively coarse grit sizes of about 120 and coarser since, with finer grits, the grinding aid seems to have little effect. This may be because there is little space between the grits to accumulate and the layer stays essentially completely on the surface. As a result it may be removed before it has a chance to have an effect. Regardless of the theory involved, the use of grinding aids is usually confined to relatively coarse grit products.
Various materials have been proposed as grinding aids but the one that is most widely used commercially and has proved the standard for efficiency has been KBF.sub.4 or potassium fluoroborate.
The grinding aid is usually applied in a supersize layer rather than in a size layer. This is because a greater loading of the grinding aid is possible if the binder component of the composition does not also have the function of enhancing the bonding of the grits to the substrate, as is the primary function of the binder in a size coat. There is however no reason that the grinding aid could not be present in both the size and supersize layers. There are however indications that grinding aids in a size coat which is overlain with a supersize coat may contribute little to any observed improvement.
The present invention is based on the discovery of particularly advantageous grinding aid formulations. The use of these formulations, whether as supersizes, as sizes or as both, permits the realization of significant advantages over the formulations presently known.